There are many locations around the globe that have active, high velocity tidal flows. These high velocity flow areas present a great potential to convert the changing tides or tidal energy into electrical energy. Tidal turbines having large blades are placed on the ocean floor or otherwise positioned in the locations with high velocity tidal flows. The tide imparts motion on the blades of these tidal turbines which, in turn, drives a generator to produce electricity. The electricity produced is transmitted to a land-based sub-station for distribution on land. This type of electrical energy is desirable as there are many populated areas near the oceans of the world and furthermore since this source of energy is more or less invisible, beautiful ocean scenery is relatively untouched by the use of tidal turbines.
Tidal energy is to a great extent predictable. The deterministic nature of the availability of power, together with its high density and the implicit absence of visual and very minimal navigational impact makes tidal energy extraction a very attractive financial proposition particularly since virtually the whole of the available resources remain untapped.
Generally changes in current flow are due to the naturally occurring phases of the moon and sun. Tidal flows are inherently oscillatory (two directional ebb & flow). However, the ebb and flow tides are not always anti-parallel. In other words, the ebb may not be exactly 180 degrees away from the flow. In addition, the angle between ebb and flow tides can be highly spatially variable within a field. Natural structures within a tidal field may also affect the angle between the ebb and flow tides in a particular area. In addition, tidal flows change directions seasonally by 5-10 degrees. In some locations, the seasonal changes can even be more pronounced. Once local anomalies and the seasons are accounted for, the directions remain relatively predictable. Superimposed on the pattern of ebbs and flows are variations from other sources, such as intense atmospheric events
Current tidal turbines have shortcomings. Many fail to account for differences in the angle between the ebb and flow of the tide and, in this way, energy is lost because there will be a component of tidal energy in either the ebb or flow or both that will be lost. One type of turbine includes a bi-directional rotor, using symmetrical blades that operate in both ebb and flood tides. Another type of turbine include two mono-directional rotors. In these previous undersea turbines, the divergence from anti-parallel flows between the ebb and the flow is not accounted for. These systems are able to capture energy from the ebb and flow, but they are not as efficient since some of the flows do not flow directly into the turbines.
The two rotor turbine is much more expensive in that two large rotors are used; the drive train must be specifically configured to support two-directional power generation. This adds to the complexity and lowers the reliability of the turbine. In the one bi-directional turbine, the stanchion and the lack of accounting for the anti-parallel flow results in approximately a 25% extraction penalty in many environments. This type of arrangement is also less reliable. In both the ebb and flow, the flow of the tides is substantially reduced behind the stanchion. In one direction, the blade of the turbine passes through this low flow area. Each blade goes from being loaded to unloaded and then back to loaded as it passes through the area behind the stanchion. This results in fatigue stress on the blades shortens the effective life of the turbine blades. In another type of turbine, the pitch of the blades is varied like in a wind turbine. Varying the pitch also fails to account for anti-parallel ebbs and flows. If the variable pitch blades must pass in the wake of the stanchion these blades are also subject to fatigue stress. Furthermore, active movements of the blades during high flow speed times in a heavy fluid presents reliability challenges. Most of the placements of tidal turbines do not account for the non anti-parallel nature of the ebbs and flows so a component portion of the ebb or the flow or both is lost in terms of energy extraction. In other words, since the turbine is not pointed directly into the ebb or the flow or both, a portion of the potential energy that could be extracted is lost.
There is a need for an improved system capable of extracting more energy from the natural resource. In addition, there is also a need for a less expensive system and method.